The invention relates to a manipulator which, with the assistance of a control unit, forms an industrial robot for primarily picking purposes. In particular, the invention relates to a manipulator with three arms by means of which a movable member, comprised in the manipulator, is brought to arbitrary positions in space with retained orientation and inclination.
An industrial robot comprises a manipulator and control equipment, whereby the manipulator with the assistance of the control equipment carries out arbitrary operations within a working range. Usually, such a manipulator comprises a plurality of arms which support a hand, on which a tool is arranged. In the majority of robot applications, a traditional six-axis manipulator is used, which exhibits sufficient movability to carry out a wide range of operations with mostly very high accuracy. For certain applications, however, there is a need of a manipulator which has fewer degrees of freedom but which permits faster operations. Such a manipulator may be mechanically connected in such a way that the tool-carrying hand is all the time oriented in space in a predetermined manner.
From U.S. Pat. No. 4,976,582 a manipulator is previously known by means of which a movable element in relation to a stationary element may be moved in a space with retained orientation and retained inclination. The manipulator has three arms which jointly support the movable element. Each of these arms is rotatably journalled in the stationary element around a respective first axis. The arm supports a linkage, one end of which is rotatably journalled in the outer part of the arm, allowing a movement in two degrees of freedom. In its other end, the linkage supports the movable element and is journalled therein, allowing a movement in two degrees of freedom.
The linkage comprises two parallel links and is arranged, together with the arm, such that one of the degrees of freedom of each end of the linkage constitutes a rotation around an axis which is parallel to the first axis of the respective arm. Irrespective of the movements of the arm or the linkage, these three axes of rotation are thus kept in parallel. In this way, the movable element will always have the same orientation and inclination Win relation to the stationary element. One condition, however, is that the respective first axes of the arms form an angle with each other.
According to the known manipulator, the parallel links are attached with one of their ends to the respective arm and with their other end to the movable element by means of either a so,called universal joint or a pivot. A universal joint offers a movement in two degrees of freedom. A pivot, in the following text, is to be understood as a centre-point bearing. Such a bearing fixes two objects to each other with three degrees of freedom. It thus provides a tilting movement in two directions while at the same time providing rotation. A common embodiment of a pivot is a so-called ball joint, which is a bendable and turnable connect-ion between two objects, for example links. The end of one of the links is thus formed as a ball, which for the most part is surrounded by a cup in the end of the other link.
Two parallel links with pivot attachment offer a movement with two degrees of freedom. A single link with a universal joint also offers a movement with two degrees of freedom although the accuracy in the movement becomes greatly deteriorated. Providing two parallel links with a universal joint each thus results in the system becoming overdetermined. This may result in stresses which wear on the bearings. Parallel links with pivot attachment are thus preferable.
To increase the movability of the manipulator, in a ball-cup bearing, the actual cup must be arranged with a small spherical extent. However, if the cup is arranged to be smaller than half a sphere, the joint can no longer keep together. An external force must then all the time press the ball and the cup against each other such that the joint is capable of transferring compressive and tensile forces without any play arising. In this context, U.S. 5,333,514 teaches a manipulator comprising parallel links, between each pair of which a spiral spring is arranged. The two links each comprises a cup, which cups are facing each other and are each adapted to fit over a respective ball. The tensile force between the parallel links, arising through the spiral spring, then ensures that the ball and the cup are in contact with each other with such a force that the cup does not jump off the ball at accelerating movements.
The known spiral spring, which in this way exerts a tensile force, has proved to have limited strength. The hooks, arranged at the ends of the springs, are subjected to such repeated load changes that fatigue causes fracture. Further, the known spring, in order to be placed in position, must be stretched out and then hooked on. It then often happens that this stretching results in the yield point of the material being passed, whereby the resilient properties are deteriorated such that the spring must be rejected. A further disadvantage with the known spring, in connection with operations in a hygienically sensitive environment, is that the spiral spring attracts dust and particles which are difficult to clean away.
The object of the present invention is to suggest ways and means of manufacturing a manipulator of the kind described above, which comprises a spring device which exhibits improved strength and reduced risk of elongation. From a second aspect, the spring device shall also have a reduced tendency to attract dust and, in addition, be easy to clean and thus useful for operations with high hygienic requirements.
These objects are achieved according to the invention by a manipulator according to the characteristic features described in the characterizing portion of the independent claim 1 and with a method according to the characteristic features described in the characterizing portions of the independent claims 6 and 7. Advantageous embodiments are described in the characterizing portions of the dependent claims.
According to the invention, the tensile force between the parallel links of a manipulator of the kind described above is provided by a spring device where the spring force is obtained by compressing a spring instead of stretching it out, as is known. This is attained by allowing two rods provided with pistons to cross each other and arranging a spring between the pistons. When applying a tensile force between the free ends of the rod, the two pistons are moved towards each other, the spring arranged between the pistons thus being tensioned by compression. One of the rods may equally be replaced by a clamp means surrounding the spring. Any member exhibiting a spring force corresponding to pressure may be arranged as a spring.
According to a first aspect of the invention, the spring device comprises a sleeve-shaped housing in which a spiral spring is arranged. At one end the housing is open, and at the other end it is provided with a bottom, against which the spiral spring rests. In its open end, the housing is provided with a fixing device for attachment to one of the parallel links. A rod provided with pistons is arranged through the housing and the spring and penetrates the sealed end of the housing. The free end of the rod is arranged with a fixing device for attachment to the other parallel link. During mounting, the two fixing devices are pulled from each other, whereby the spiral springs are brought to be compressed between the piston and the bottom of the housing. The compressed spiral spring thus exerts a compressive force between the piston and the bottom of the housing, which is transformed into a tensile force in that the housing and the rod cross each other. The spiral spring may be dimensioned such that there is no risk of the spring material exceeding the so-called yield point, where the material loses its elastic properties.
In a preferred embodiment, the sleeve-shaped material is sealed also in its other end. In this embodiment the spiral spring is completely enclosed in the housing, whereby no dirt may accumulate in the spring. In this embodiment the housing is formed with smooth external surfaces and without dirt-attracting folds or pockets such that it may be easily flushed.